Add xa architecture diagram to juxtaposer README

[hughsaunders]

What does this PR do?

This diagram provides an example architecture for using juxtaposer, and may help readers to visualise the requirements.

What ticket does this PR close?

Related: conjurinc/ops#692

Checklists

Change log

• [ ] The CHANGELOG has been updated, or
• [x] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] This PR includes new unit and integration tests to go with the code changes, or
• [x] The changes in this PR do not require tests

Documentation

• [ ] This PR does not require updating any documentation, or
• [x] Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs

[codeclimate[bot]]

Code Climate has analyzed commit 5aee0352 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 53.0% (0.0% change).

View more on Code Climate.